Disposable syringes of the aforedescribed kind, by which is meant syringes that can only be used once, can be divided into a number of function-dependent categories with regard to their function-dependent characteristics.
A first syringe category is exemplified by the syringe illustrated and described in the International Patent Publication PCT/SE88/00634, International Publication No. WO 89/04677. In the case of this syringe, the rod can be twisted in relation to the container and the piston so as to move the coupling means on the one hand to a connecting state and on the other hand to a disconnecting state, as the rod is twisted. Thus, with this type of syringe, it is necessary that the piston is unable to twist or rotate or at least not essentially, at the same time as the rod is twisted in relation to the container.
It is thus of great importance with this category of syringe that means are provided to ensure that the piston will not twist together with the rod.
In a second syringe category, the rod, or red unit, carries a piston or piston unit and moves backwards and forwards in the container without twisting, and the outer surface of the piston unit and the inner surface of the container include means which mutually coact to lock the piston in a lowermost position. Examples of these prior art syringe assemblies are described and illustrated in prior publications U.S. Pat. Nos. 4,391,272; 4,775,364; PCT/GB87/00728 with respective International Publication Nos. WO 88/02640; GB-A 2,197,792 and FR-A 2,613,628.
A third category of disposable syringes includes a rod unit which is moved forwards and backwards in the container without twisting, and a piston unit which is fastened to the rod unit by means of a separate connecting part.
This connecting part is pivotally mounted on the rod part for rotation about a centre line on the rod, the piston and the container, and can be guided into to different rotational positions by grooves disposed on the piston unit.
An example of this type of syringe is described and illustrated in publication NO-A 163,263.